The present invention relates generally to the use of glass, polymer, crystal or silica-based photonic band-gap materials to create Photonic Band-gap Light Emitting Fibers (PBGLEF).
The technology is based on the ability of periodic dielectric structures i.e., Photonic Band-gap (PBG) structures, to modify the radiative properties of light sources inside the structures.
The unique properties of a PBG permits the fabrication of scintillating light and fluorescent light optical fibers, for use in image and radiation detection devices laser, amplifier, and other optical device applications, with light-trapping efficiencies greater than that obtained with conventional fibers.
Conventional scintillating light optical fibers typically consist of a glass core covered with a cladding material where the refractive index of the core material is higher than the refractive index of the cladding material. The mismatch of refractive indices sets the condition for total internal reflection within the fiber to trap scintillating light, and gives the fiber its wave guiding properties. Typically, a multimode fiber traps less than 4% of the scintillating light contained within its total reflection cones.
The additional photons guided within multi-clad fibers increases the output signal over conventional single clad fibers, bringing the total light-trapping efficiency up to 6%.
Clearly, a need exists to improve the efficiency of conventional optical fibers to increase the fiber""s light trapping efficiency.
Photonic crystals, having periodic dielectric nanostructures that produce photonic band-gaps, are unique materials that affect the density of electromagnetic states within their boundaries. They can affect the radiative dynamics within their structure to change the properties of optical devices fabricated from these materials. The concept of photonic band-gap materials, first proposed by Yablonovich, xe2x80x9cPhotonic band structuresxe2x80x9d, J. Mod. Opt., Vol. 41, pp. 171-404, 1994 is well known in the art and has stimulated a large amount of theoretical and experimental work.
Electronic waves traveling in the periodic potential of a crystal are arranged into energy bands separated by gaps in which propagating states are prohibited. When a wave propagates in a periodic structure (in any number of dimensions), the dispersion curves that relate the frequencies of the wave to the wave vector, characterizing its propagation, possess a number of branches. These branches form bands that are separated by frequency gaps at points of symmetry in the corresponding Brillouin zones (i.e., a frequency range in which no wave can propagate) that exist for all values of the wave vector in the Brillouin zone. This also gives rise to a gap in the density of the states of the waves propagating through the structure. Clear PBG-induced gaps in the emission spectra of internal sources have been observed as reported in the work of E. P. Petrov, V. N. Bogomolov, I. I. Kalosha, and S. V. Gaponenko, xe2x80x9cSpontaneous emission of organic molecules embedded in a photonic crystalxe2x80x9d, Phys. Rev. Lett., Vol.81, pp. 77-80, 1998.
PBG materials modify the spontaneous emission rate of excited atoms and spontaneous emission is inhibited when the imbedded atom has an emission frequency in the band gap. The absence of electromagnetic modes inside a photonic band-gap permits atoms or molecules imbedded in a dielectric crystal to be locked in an excited state if the energy of this state, relative to the ground state, falls within the photonic band-gap.
The localization properties of photons in a PBG structure are similar to those of electrons in the band-gap of a semiconductor. For frequencies within the photonic band-gap, photons incident from outside the material will be perfectly reflected.
A dielectric crystal that reflects electromagnetic waves incident from any angle can be fabricated. Three-dimensional dielectric structures have been found to exhibit a photonic band-gap. In practice, however, these dielectric crystals have complex structures and their fabrication is difficult to accomplish.
Two-dimensional lattice structures however, will behave as a PBG for in-plane incident waves and the simplicity of the geometry facilitates fabrication of these dielectric crystals.
Photonic band-gap structures comprise a periodic lattice of materials of one dielectric with a high refractive index imbedded in another material of low refractive index, where the inclusions act like atoms in a crystalline structure. Arrays of dielectric material can be periodically arranged along two axis (x and y), and be homogeneous in the third direction (z).
In accordance with the principles of the present invention, controlled nanometer variations in the refractive index of PBG crystals are created to fabricate highly ordered nanostructures (10 nm to 1000 nm) in an environmentally robust glass polymer or crystal matrix. The invention comprises two-dimensional PBG structures that are composed of a plurality of regular high refractive index inclusions, instead of the xe2x80x9cair holesxe2x80x9d typically employed in PBG fibers.
The invention uses PBG structures to limit the manifold of emission modes to those that emit photons only in the axial direction (i.e., parallel to the length of the fiber).
Fibers designed with the two-dimensional PBG structures of the invention can overcome the current limitations of existing scintillating fiber and fiber amplifier technologies by modifying the spontaneous emission rate of excited atoms having an emission frequency in the band gap. As such, a light-trapping efficiency at least one order of magnitude higher than that of a standard fiber can be achieved.
Using the principles of the invention, large refractive index steps can be obtained in an all solid-state fiber. In addition, large diameter single-mode core fibers can be fabricated with a strong and well-controlled shift in dispersion to guide light in a single propagation mode and dramatically lower transmission losses. Further, the PBG structure of the invention limits photon emission to directions collinear with the fiber core to increase fluorescence and scintillation collection efficiency.
The two-dimensional PBG""s of the invention permits the fabrication of high efficiency light-trapping fluorescent and scintillating light fibers, for improved performance in fiber amplifiers, lasers, and detectors. Additionally, essentially lossless devices can be designed into fiber structures to reduce significantly the interaction length required for amplifiers or other devices (such as couplers and splitters).
In one embodiment of the invention, an all-silica photonic band-gap structure is used to fabricate a high-efficiency, scintillating light fiber. In another embodiment of the invention, an all-silica, rare-earth doped photonic bandgap structure is used to fabricate a high-efficiency, large diameter, single mode fluorescent fiber.